Iranian authorities have released 140 people detained during a crackdown on protesters following last month's disputed presidential election.

The detainees were freed on Tuesday during a visit to the Evin prison in the capital, Tehran, by a parliamentary committee investigating prisoners' conditions.

The protesters were among at least 500 people believed to have been arrested during demonstrations that erupted after the June 12 vote, which resulted in the re-election of Mahmoud Ahmadinejad as the country's president.

Kazem Jalili, a spokesman for the committee, said another 150 people remain in detention across Iran and could be released soon, according to the state-run IRNA news agency.

Also detained are 50 "political figures and members of counter-revolutionary or foreign groups who meddled in the riots", Jalili said.

Reports of abuse

The move on Tuesday followed an order from Ayatollah Ali Khamenei, Iran's supreme leader, to close a second detention centre where opposition protesters were detained.

Alireza Ronaghi, Al Jazeera's correspondent in Tehran, said the parliamentary committee was initially motivated to launch its investigation after reports of mistreatment and abuse of the prisoners.

"But what appears to have been the main reason for the investigation was the death of the son of a highly respected conservative politician," he said.

"Mohsen Rouhalamini, who was arrested on July 9, was said to be killed in prison. Authorities said he had died from meningitis, but there were reports that his teeth were smashed.

"The news was taken very seriously by the conservatives as it revealed that the violence was spiraling out of control."

Although the protests have largely been quelled, political tensions continue to run high in Iran with the defeated candidates refusing to accept the result.

Mir Hossein Mousavi and Mehdi Karroubi, two defeated presidential candidates, say the vote was rigged.